Add (SOCK_DGRAM, IPPROTO_UDP) sockets to the sandboxing whitelist

If we did not find a non-private IPaddress by iterating over interfaces,
we would try to get one via
get_interface_address6_via_udp_socket_hack().  This opens a datagram
socket with IPPROTO_UDP.  Previously all our datagram sockets (via
libevent) used IPPROTO_IP, so we did not have that in the sandboxing
whitelist.  Add (SOCK_DGRAM, IPPROTO_UDP) sockets to the sandboxing
whitelist.  Fixes bug 19660.